Suction rollers of the type in question are used, for example, to convert a torque into a tractive force in webs of material in coating machines, for example for coating photo paper, film or magnetic tape, when the static friction between the transfer roller and the web is inadequate.
In known suction rollers in which the sealing element arranged in the suction roller internally covers that part of the roller around which the web is not looped with a slight overlap, air leaks through the perforation of the casing into the sealing gap between the casing and the sealing segment and causes the web to flutter undesirably at the points where it runs onto and leaves the suction roller, so that the web is not smoothly transported. To eliminate this deficiency, DE 14 74 973 proposes an inner ring element consisting of two chambers in which that part around which the web is looped is under reduced pressure while the other part is under atmospheric pressure, reductions in cross-section being provided between the two chambers.
It is already known from GB-PS 983 951 that the roller itself can be provided with a permeable casing. This casing remains partly free during the transport of a web of material so that, because the entire periphery of the roller casing has to be placed under vacuum, an unusually high vacuum and, hence, considerable energy are necessary. For this reason, mask arrangements are proposed for peripheral adaptation to the width of a web. This is complicated and does not exactly cover the peripheral regions. In addition, the effort involved in operation is considerable, the guiding of the web, particularly at its borders, is unstable and, because of the accessibility, dirt is sucked in. In addition, problems arise in regard to the vacuum to be applied because no indentations should be formed in the covered part of the roller casing which consists of porous material, for example sintered material. Apart from this, however, that part of the roller casing around which the web of material is not looped is always free so that the porous roller casing gradually becomes blocked. Periodic cleaning is therefore necessary and necessitates dismantling of the suction roller arrangement.
These disadvantages are said to be avoided by the suction roller arrangement described in DE 31 11 194. In this suction roller arrangement, longitudinal grooves are arranged at intervals from one another in the outer surface of the cylindrical roller and are covered by strips of air-permeable material. At its ends, the roller is covered by wear disks which are fixed to the roller and onto each of which a sealing ring is pressed under spring tension. The inner space beneath the looping angle of the web-form material is under reduced pressure while the space around which the web is not looped is fed through openings in the sealing rings with compressed air which lifts the web of material off the roller and, at the same time, is intended to blow dirt which has been sucked in out of the porous strips. However, an arrangement such as this is complicated and therefore expensive to make and, in addition, has the disadvantage that heavy wear can be expected at high rates of travel which shortens operating times and increases production costs.